Coordinated Multi-Point (CoMP) transmission has been applied in a Long Term Evolution-Advance (LTE-A) system to thereby lower interference to a User Equipment (UE) at the edge of a coverage area of a small cell from an adjacent cell, so as to improve an experience of the UE at the edge of the cell. Coordinated Multi-Point (CoMP) transmission refers to cooperation among a plurality of Transmission Points (TPs) separate in geographical position. Typically the plurality of transmission points refer to eNBs of different cells, or an eNB of a cell and a plurality of Remote Radio Heads (RRHs) controlled by the eNB. CoMP transmission can be categorized into downlink coordinated transmission and uplink joint reception. Downlink coordinated multi-point transmission is generally further categorized into two transmission schemes: Coordinated Scheduling/Coordinated Beam-forming (CS/CB) and Joint Processing (JP). In the CS/CB scheme, one of the plurality of transmission points transmits a useful signal to the UE, and interference from the other transmission points to the UE is lowered as much as possible through joint scheduling and beam-forming. The joint processing scheme can be further categorized into schemes: Joint Transmission (JT) and Dynamic Point Selection (DPS). In the JT scheme, the plurality of transmission points transmit useful signals to the UE concurrently to thereby enhance the received signal of the UE. In the DPS scheme, the transmission point to the UE is switched dynamically by always selecting the optimum one for the UE among the cooperating transmission points to transmit a signal to the UE. These schemes of coordinated multi-point transmission can be applied in combination with each other or can be combined with Dynamic Blanking (DB) to dynamically disable some transmission points from transmitting signals over some time-frequency resources.
Downlink coordinated multi-point transmission is implemented based upon Channel State Information (CSI), obtained by the eNBs, of the UE to the respective cooperating points. The CSI includes a Channel Quality Indicator (CQI), a Pre-coding Matrix Indicator (PMI), a Rank Indicator (RI), etc. The UE measures channels of the respective eNBs to the UE using downlink reference signals transmitted by the eNBs, and feeds channel information measured by the UE to a serving cell of the UE. The serving cell of the UE receives the CSI and performs coordinated scheduling and/or coordinated pre-coding with the cooperating cells to thereby implement coordinated transmission.
Existing schemes of distributed coordinated scheduling can enable inter-cell coordinated scheduling and/or coordinated pre-coding scheduling, as illustrated in FIG. 1.
Distributed scheduling is performed respectively at the respective cooperating eNBs without exchanging a large amount of CSI via an X2 interface, so that the amount of information transmitted via the X2 interface may be less than in centralized scheduling, as illustrated in FIG. 2. However distributed scheduling can not optimize a global scheduling result according to scheduling conditions between the eNBs. In order to achieve a nearly globally-optimized result, iterative scheduling between the eNBs may need to be performed so that the cooperating eNBs may need to exchange the scheduling information with each other repeatedly if there is a significant delay via the X2 interface between the eNBs, then repeated exchanges of the scheduling information in distributed scheduling may come with such a high scheduling delay that the channel information may become outdated, thus degrading the transmission performance. Thus the scheme of CoMP distributed scheduling needs to be designed carefully in the scenario with a significant delay via the X2 interface to thereby minimize the number of times that the information is exchanged between the cooperating eNBs, to thereby lower the amount of exchanged information.
At present, distributed coordinated scheduling has to be performed iteratively by exchanging the scheduling information iteratively so that there may be a considerable increase in scheduling delay via the X2 interface with a significant delay.